1. Technical Field
The invention relates to a device to determine the absorption in a sample that includes:                a) An incoherent emission source to create a measuring light beam;        b) A resonator with at least two mirrors to which the measurement light beam may be coupled as input;        c) A sample volume to receive an absorbing sample within the resonator; and        d) A detector to receive the beam that may be coupled to the output from the resonator.        
The invention further relates to a method to operate such a device.
2. State of the Art
A method to measure the wavelength-dependent absorption of a material is known from classical absorption spectroscopy. In such a method, the sample is penetrated with white light. White light (continuum irradiation) consists of a so-called wavelength continuum, i.e., in the wavelength spectrum under consideration, irradiation occurs at all wavelengths. Contrastingly, monochromatic light is understood to be irradiation that is limited to a very narrow wavelength range, i.e., to consist of only one ‘color.’ In classical absorption spectroscopy, the light is split into varying wavelengths behind the sample by means of a monochromator. The resulting spectrum may be received using horizontal or surface detectors at the output of the monochromator. The sample does not absorb equally at all wavelengths. The absorption spectrum of the sample may be determined by measuring the irradiation with and without the sample, or with the help of a reference beam that passes through a reference medium without absorption. The molecules present in the sample may be identified based on the received absorption spectrum, e.g. by comparison with known molecular spectra. Further, the quantity of the molecules present in the sample may be determined. This is done by determination of the absorption coefficients that are dependent on the concentration and the wavelength-dependent extinction of the substance being observed. This known method is simple and low-cost, but provides very low sensitivity.
A method to measure atmospheric trace gasses is known as “DOAS” (Differential Optical Absorption Spectroscopy). In this method, the white light from a lamp is directed through the atmosphere for a distance of a few meters to several kilometers. A reflector reflects the irradiation toward a telescope, and the spectrum is received by a monochromator and a horizontal detector. When very long absorption paths are used, very low concentrations of trace gases may be determined. The disadvantages are low spatial resolution, long integration times at the detector, and the dimensions of the measuring facility. This makes the method expensive and inflexible.
For example, multiple-path cells (so-called multi-pass cells—MPAS) are known from the publication “Very long paths in air” by J. U. White in J. Opt. Soc. Am. 66, 411, of 1976. In them, continuum irradiation is guided through the sample using mirrors whereby the absorption path length is increased. This achieves an improved level of sensitivity. MPAS still suffers, however, from deficient sensitivity and unfavorable signal-to-noise ratio because of the loss at each mirror reflection. Further, there are problems in the practical realization of numerous transfixing paths. Because of the normally strong divergence from incoherent light sources, only a few transfixing paths are possible. Even if lasers are used, the number of transfixing paths is limited to about 100. The systems are also mechanically susceptible to inaccuracy because even the slightest de-adjustment of the mirrors causes a large alteration to the irradiation path.
Methods using lasers as coherent, monochromatic emission sources are known as Cavity Ring-Down Spectroscopy (CRDS) and Cavity Enhanced Absorption Spectroscopy (CEAS). In both methods, the laser light is coupled to the input of an optical resonator. A resonator consists of at least two mirrors with high reflectivity. These are particularly known in laser technology. The coupled light reflects within the resonator, and forms so-called modes at the resonator wavelength, i.e., standing waves of minima and maxima. The modes are not formed at other wavelengths.
The simplest resonator consists of two parallel planar/concave mirrors whose reflective surfaces face each other. There are also ring resonators consisting of several mirrors. At one of the resonator mirrors, light is decoupled out of the resonator and directed to a detector for measurement, where the measurement signal is created.
In the CRDS method, an absorbing sample is placed into the resonator. A laser pulse from a pulsed laser beam is coupled into the resonator. Because of reflection losses and losses upon output, the laser pulse light reflected, or stored, within the resonator becomes weaker, and the signal received at the detector is reduced. The temporal signal progression follows an exponential function with decay time τ0 if the sample does not absorb. Additional light losses occur within the resonator in the presence of an additional absorption by the sample at this wavelength. The decay time then decreases to a lesser value τ<τ0. The absorption coefficient, and thus the sample quantity, may be determined from the decay time. This method requires reception and evaluation of a correspondingly temporally highly resolved signal by the detector. Further, a laser pulse with corresponding high intensity must be created in order to be capable of being registered at the detector, and to create a favorable signal-to-noise ratio.
The decay curve of the laser pulse intensity must be reproducible. Correspondingly high demands are placed on the individual components, which make the method more expensive. It is particularly disadvantageous that the absorption coefficient is measured only at one wavelength. In order to be able to measure at, other wavelengths, the laser must be adjustable, and the wavelength must be scanned, which requires much time.
The CEAS method is known from the publication “Cavity Enhanced Absorption and Cavity Enhanced Magnetic Rotation Spectroscopy” by R. Engeln, G. Berden, R. Peeters, and G. Meijer in Rev. Sci. Instr. 69, 3763, dated 1998. In it, a continuous laser (cw laser) is used instead of a pulsed laser. The wavelength of the laser is continuously determined about that of the resonator over the entire spectrum. At the “correct” wavelength, the light is coupled into the resonator in a defined manner, and can thus build modes within the resonator, as with the CRDS method. If a sample that absorbs at this wavelength is located within the resonator, the reciprocally, temporally integrated, transmitted light intensity is proportional to the absorption coefficient of the sample. In other words, simple integration of the signals over time may be used to determine the concentration of the material in the sample. This method also operates only at one wavelength with narrow-band lasers. Moreover, regularly occurring mode jumps in diode lasers represent a technical disadvantage for this method. Even so-called Intra-cavity Spectrometry, in which the measurement light beam is created by a laser-active medium within the resonator, functions at only one wavelength, or is limited by the bandwidth of the laser.
A CRDS method is known from the publication “A Fourier Transform Cavity Ring Down Spectrometer” by R. Engel and G. Meijer in Rev. Sci. Instr. 67, 2708, dated 1996, in which pulsed lasers with wider bandwidth are used instead of extremely narrowband lasers. A Fourier transform spectrometer positioned after the resonator allows the required wavelength selection. The color lasers used complicate the operation of the method, and are not useable for simple or small devices. Even diode lasers are not well suited for this application because of regularly-occurring mode jumps.
Similarly, wideband, pulsed color lasers are used in the device described under the title “Pulse-Stacked Cavity Ring-Down Spectroscopy” by E. R. Crosson, P. Haar, G. A. Marcus, H. A. Schweitmann, B. A. Paldus, T. G. Spence, and N. R. Zare in Rev. Sci. Inst. 70, 4, dated 1999. The method described disperses the received laser pulses by means of a monochromator before they are received. The configuration described includes a large number of components, and is therefore expensive as well as awkward to operate because of the color lasers used.
The publication “Integrated Cavity Output Analysis of Ultra-Weak Absorption” by A. O'Keefe in Chem. Phys. Lett. 293 (1988), p. 331, describes a configuration in which a pulsed color laser is used to create the measurement light beam for CRDS whose signal is integrated with respect to time. This structure is also awkward because of the color lasers used.
A configuration is known from the publication “Fourier Transform Phase Shift Cavity Ring Down Spectroscopy” by E. Hamers, D. Schramm, and R. Engeln in Chem. Phys. Lett. 365 (2002), pp. 237-243, in which light from a Xenon arc lamp is coupled into a resonator and then the phase shift caused by the sample is measured with the help of a Fourier transform spectrometer to determine absorption. Such a configuration is expensive and susceptible to mechanical disturbance since the light intensity, for example, must be modulated by high frequency before entry into the resonator. Also, the range of spectrum that may be sampled is limited.